1A verb-forming suffix attached to the stem of an i-adjective or na-adjective, indicating that a third person shows outward signs of a feeling or sensation. The result conjugates as a godan verb — 寒い → 寒がる, 欲しい → 欲しがる, 痛い → 痛がる. In Japanese, subjective adjectives such as 寒い, 痛い, 欲しい, and 恥ずかしい normally apply only to the speaker; to describe someone else's inner state, you attach 〜がる to indicate that the person is showing signs of that feeling (e.g., 彼は寒がっている — 'he looks/seems cold').
子どもはお店に行くと、すぐに新しいおもちゃを欲しがる。
Whenever we go to a store, the child immediately wants a new toy.
弟は注射を怖がって、診察室に入りたがらない。
My little brother is afraid of injections and doesn't want to go into the exam room.
彼女は人前で話すのを恥ずかしがって、なかなか手を挙げない。
She gets shy about speaking in front of people and rarely raises her hand.
猫は水を怖がることが多い。
Cats often show fear of water.
祖母は冬になるとすぐに寒がって、家の中でも厚着をする。
Once winter comes, my grandmother gets cold easily and wears thick clothes even indoors.
2A suffix attached to an adjective stem (or sometimes a noun) meaning 'to behave as if one were 〜' or 'to put on a show of being 〜', even when that is not one's true feeling. Common examples: 強がる ('to act tough'), 得意がる ('to act all proud'), 偉がる ('to put on airs'). Carries the nuance of pretense or show, from the speaker's viewpoint.
彼は怪我をしているのに、強がって平気な顔をしている。
Even though he's hurt, he's putting on a brave face and acting like nothing's wrong.
あの人はいつも得意がって自慢ばかりするから、付き合いづらい。
He's always putting on airs and bragging, which makes him hard to get along with.
強がるのはやめて、つらいときは素直に助けを求めていいんだよ。
Stop putting on a brave front — when things are hard, it's okay to honestly ask for help.